Due to the continuing depletion of fossil fuels, the emerging effects of CO2 emissions, and the rising demands for energy, there is a greater need than ever for alternatives to traditional fossil fuels. The relatively high rate of waste production is another problem the world must grapple with. Waste management has become an increasingly complex matter as improvements in technology and recycling schemes are often not sufficient to counter growing waste production, obsolescence of existing waste management facilities, and shortage of space for the construction of new facilities.
Agricultural waste, biological waste, municipal sewage sludge (MSS), municipal solid waste (MSW), and shredder residue are amongst the types of waste being produced today. Agricultural waste, which includes waste from the food processing industry and agricultural industry, typically contain large amounts of water and are perishable, generating malodorous fumes in the process. When this type of waste is usually discarded, the deposit of these substances as landfill results in their decay, producing large amounts of nitrate/nitrite and methane gas which can then contaminate groundwater. Alternatively, such materials are sometimes incorporated into animal feed, thus potentially passing on pathogens and maintaining other undesirable characteristics in the food chain.
Proper management, handling, and disposal of biological waste are also imperative in the face of increasing population density. Nationally, hospitals are the major generators of medical waste, producing in excess of 500,000 tons each year in the United States. Many states concerned with the growing threat of Acquired Immune Deficiency Syndrome (AIDS) have caused more and more articles and materials to come under the definition of medical waste, which is expected to more than double the amount of medical waste being generated. The health and environmental dangers posed by biological waste mandate that special collection, transportation and disposal techniques be developed.
Municipal sewage sludge (“MSS”), by virtue of its origin, contains a large percentage of human waste and thus a high concentration of phosphates and nitrates, which are desirable components of fertilizer. However, the industrial wastes present in the sewage leaves highly toxic materials such as industrial solvents, heavy metals, behind in a sludge. When applied to the fields, the sludge releases both nutrients and high concentrations of toxic chemicals to the environment. Live pathogens also remain in the sludge and, when propagated, contaminate the soil and leach into groundwater. Disposal of the sludge is expensive and normally constitutes up to 50% of the total annual costs of wastewater treatment. The major sludge disposal options currently used include agricultural utilization, landfill, and incineration.
Wastewater treatment plants currently are designed to minimize sludge production and all efforts are taken to stabilize and reduce its volume prior to disposal or utilization. Furthermore, increasing sludge disposal costs and diminishing landfill capacities are continually driving interest in sludge drying. Although drying reduces the bulk and weight of sludge, thereby lowering the transport and disposal costs, it is a very energy intensive and expensive process. While numerous sludge processing options have been proposed and have the potential to convert a fraction of organic material into usable energy, only a few have been demonstrated to have a net energy yield at full scale.
Generally, municipal solid waste materials are landfilled and/or incinerated. Environmental restrictions on both landfills and incinerators demand that an alternative solid waste solution be implemented. The public outcry concerning pollution caused by incinerators has also halted construction of many new incinerator projects.
Treatment of industrial waste, namely shredder residue, likewise presents another challenge. Shredder residue generally consists of the nonmetallic content of the automobile and other materials (and their constituents), such as air conditioners, refrigerators, dryers, and dishwashers, the latter products being commonly known as white goods. The shredder industry recovers about 10-12 million tons/yr. of ferrous scrap, most of which is from shredded automobiles. However, for each ton of steel recovered, about 500 lbs. of shredder residue is produced. While many components of end-of-life automobiles, household and commercial appliances can be recycled, reused, or recovered, a significant portion is left over from the shredding process and finds its way into landfills. Disposal of shredder residue is made all the more difficult by the toxic materials found therein, e.g. cadmium, lead, mercury, and other heavy metals. Due to the limited amount of space available for landfill use and the increasing costs of hazardous waste disposal, an alternative solution is needed. The automotive and recycling industries are currently under pressure to devise ways of using shredder residue in a cost-effective and energy-efficient manner.
Although a number of waste management methods are currently employed, they are either impractical, generate further pollution, or are too costly in terms of energy and economics. Some of these methods include composting, incineration, disposal as landfill, agricultural application, and dumping at sea. As indicated in Table 1 below, each method is beset by various drawbacks.
TABLE 1Prior Art DrawbacksCompostingWarehousingLandfill DisposalAgricultural UseMarine DumpingPathogenLimited SpaceLimited SpaceHeavy MetalMarine LifeContaminationAvailableAvailableBuildupPoisoningHaulage/TransportLeaching intoDiseaseCostGroundwaterTransmissionGreenhouseHaulage/EmissionsTransport CostHaulage/TransportCost
Other recycling approaches to waste management, including incineration, biotreatment, pyrolyzers, and gasification have their own attendant problems. As case in point, biotreatment in the form of aerobic and anaerobic digestion requires long holding times, strict monitoring and control of operating conditions, e.g. oxygenation, pH, temperature, etc. for the selected microbes, specialized equipment, and generally results in non-uniform treatment and final products filled with pathogens. Additionally, bacteria that may have been developed to consume specific compounds will, when exposed to the waste substrate, activate alternative enzyme systems to consume other more easily processed compounds.
Incineration/combustion involves the use of equipment and parts to comply with toughened emission regulations. Large volumes of gas are produced and must be disposed of using large specialized equipment. Most conventional systems cannot process a variety of waste substrates, such as solid waste, which would oxidize too high up in the furnace, or high-moisture feedstocks, for which a tremendous amount of energy must be expended to remove the water content. As such, there is a great heat/energy loss to the system.
Pyrolyzers have been used to break down organic matter to gas, oils and tar, and carbonaceous materials. A pyrolyzer typically heats organic materials at high temperatures, about 400-500° C., with poor energy efficiency and little, if any, control over the product composition. Most waste materials, especially agricultural waste, are high in moisture. As with incineration, pyrolysis aims to boil off the water using a very energy intensive process. The typically large holding vessels used in pyrolysis results in significant interior temperature gradients, non-uniform waste treatment, and yields contaminated end products.
Gasification achieves a partial combustion of waste materials but, like pyrolysis, does not operate efficiently with wet waste as energy is expended to remove water from the feedstock. There is little control over the type or composition of products due to non-uniform treatment of the feedstock and the principal usable energy-containing products are gases that are not as useful as other products. Traditional thermal oxidation treatments also produce noxious gases and dioxins.
Both the products of pyrolysis and gasification methods, respectively, can contain unacceptably high levels of impurities, e.g. tar, asphalt, and have low calorie content. For instance, sulfur- and chlorine-containing waste yields sulfur-containing compounds, e.g., mercaptans, and organic chlorides in the end products. Typically, chlorinated hydrocarbons at levels of 1-2 ppm can be tolerated in hydrocarbons, but neither gasification nor pyrolysis methods can achieve such low levels with any reliability. Poor heat transfer, nonuniform treatment, and an energy intensive water removal process have generally limited pyrolysis methods and gasification approaches to only about 30% energy efficiency.
In recent years, methods as disclosed in U.S. Pat. Nos. 5,269,947, 5,360,553, and 5,543,061, have been developed to attempt to produce higher quality and more useful oils. However, such processes can have drawbacks. For example the disclosed processes may not adequately handle sulfur- and chlorine-containing compounds, or efficiently process wet waste substrates due to significant energy requirements and thus have not been widely commercialized. As illustrated by the foregoing, there remains a need for sustainable recycling processes that are sound from a technical, economic, and environmental perspective.